While I don't want this to be political, the reality is actually largely political.
While both Hong Kong and Singapore were former British colonies, so it is not a surprise that their designs have been similar. But what makes difference was Lee Kuan Yew. As a controversial politician, Lee's plan for Singapore has been largely focused on the long-term, while many plans for Hong Kong has been short-term.
To Singapore, hawker centres are permanent establishment. Even Singapore has helped hawkers to promote this kind of settings. On the other hand, hawker centres in Hong Kong are generally temporary establishment, even having permanent fixtures, solely aiming to reduce hawkers on the streets and improve public hygiene.
However, HK is different -- each centre seems to have a concentration of only 6-7 larger actual restaurants with more full service, and people order everything from that one place. Little/few sampling going on. Also, air conditioning.
I don't know which one you have been. But most likely they are food courts rather than any hawker centres.
Is it because HK already has many street level small food outlets, while Singapore not so many so they flock to hawker centres?
Actually, it is simple - marketing. Street-level stores attracts more food traffic.
Street-level stores